1. Field of the Invention
This invention relates to a water purifier, and more particularly, to a water purifier for providing high quality reagent grade water with a resistivity of up to 18.3 megohm-cm and a total organic carbon content on the order of 10 parts per billion.
2. Description of the Related Art
With the increasing sophistication of scientific testing and analysis, the requirements for uncontaminated water for laboratory use has likewise increased. The purity of water is graded according to standards established by professional societies. For example, Type I water is the highest purity and is used for high performance liquid chromography, atomic absorption spectrometry, tissue culture, etc. Type II water is less pure and may be used for hematological, serological, and microbiological procedures. Type III water is suitable for general laboratory qualitative analyses such as urinalysis, parasitology and histological procedures.
The majority of impurities in potable water are in the form of disassociated ionized mineral salts which carry an electrical charge; and therefore, the presence of ionic contaminants is directly related to the electrical conductivity of the water. Conversely, the absence of the water's ability to conduct electricity is measured by its resistivity. Therefore, conductivity and its reciprocal, resistivity, are standard variables by which the purity of water is measured. Typically, resistivity is measured in ohm-cm units at a reference temperature of 25.degree. C.; and at that temperature, absolutely pure water measures 18.3 megohm-cm.
More recently, high performance liquid chromatography ("HPLC") has been used to detect the presence of low levels of specific organic compounds. Consequently, water used in that analysis should contain no more than a few parts per billion ("ppb") of total organics.
In order to achieve the above standards of water purification, existing water purifiers are relatively large and typically have four or more large canisters containing purification cartridges. In those systems, water enters the canister at the top, typically flows down through the cartridge, exits the cartridge at the bottom of the canister, and flows up through the canister on the outside of the cartridge. Such a flow path exposes the water to a substantial surface area after passing through the purification media thereby adding contaminants to the purified water. To deal with that contamination, more purification media is required. While the reusable canisters have a sturdy construction, the canister cartridges have a less substantial construction which may permit purification media fines to escape the cartridge and lodge in downstream components.
In other water purification systems, several purification cartridges are packaged together into a single purification unit which via fluid couplings may be plugged into and unplugged from the purifier. A disadvantage of such a system is that cartridge elements within the purification unit may have different life cycles; and therefore, the unit life is determined by the cartridge element having the shortest life cycle.